1. Technical Field
The present invention generally relates to a stator for an electric rotating machine, such as an alternator or a motor-generator, installed in a vehicle, such as an electric vehicle or a hybrid vehicle, and to a manufacturing method of the stator.
2. Related Art
A stator for a vehicular electric rotating machine generally includes a stator core disposed opposed to a rotor and a stator winding provided at the stator core, as disclosed in JP-A-2001-204151. The stator winding in such a stator is made up of a plurality of conductor segments each having an inner conducting body accommodated in a slot of the stator core and coil end portions axially exposed from the slot and extended in both circumferential directions. Each conductor segment has a turn portion and slant portions that cross other conductor segments in a coil end. Each slant portion has a tip end which is provided with a weld portion. The coil end portions of the respective conductor segments are annularly disposed being substantially equally spaced apart from each other, with a plurality of weld portions of different conductor segments being joined to each other to provide a joint. Each of such joints is coated with an insulating resin material.
Similarly, in a vehicle alternator disclosed in JP-A-2000-60051, a predetermined number of pairs of a plurality of conductor segments are welded at coil end portions to form a winding. Each weld portion and the slant portion adjacent to the weld portion are coated with a resin material to achieve insulation between the weld portions, and between the weld portions and a perimeter frame. In addition, the annular provision of the resin material enhances rigidity at the weld portions of the stator and thus contributes to reducing vibration.
The coating such as of the weld portions of a stator is provided by a fluidized-bed coating process. In the fluidized-bed coating process, a powdered resin is located in a vessel and air is supplied to the powdered resin for agitation. Then, preliminarily superheated weld portions of a stator are immersed in the agitated powdered resin to melt the powdered resin with the heat of the weld portions to thereby provide coating on the surfaces of the weld portions. The fluidized-bed coating process is an organic solvent-free process and thus has an advantage of creating little environmental damage and having no coating loss in recovery and recycling.
However, the vehicle alternators as disclosed in JP-A-2001-204151 and JP-A-2000-60051 tend to use higher voltage (e.g. increased from 14 V to 42 V) in order to reduce weight and achieve higher efficiency in the vehicle wiring that accompanies the addition of various electrical loads and the increase of power consumption. Use of higher voltage is not limited to such vehicle alternators. For example, motor-generators installed such as in hybrid vehicles have practically come to use higher voltage for the same reason mentioned above and for increasing drive force as motors. Use of such higher voltage raises a problem of not ensuring insulation properties in the resin material mentioned above for coating the weld portions of the stator.
The reason that insulation properties are not ensured resides in the use of the fluidized-bed coating process for coating the weld portions with a resin material. In this coating process, the preliminarily superheated weld portions are immersed in the powdered resin material in a vessel. During the immersion, a number of voids (air) between the particles of the powdered resin are entrained in the film coated onto the surface of each weld portion. When some of these voids are connected through the coated film, a pin hole that allows communication between the weld portion and the outside is formed in the resin material. In a high-voltage application environment, entry of an electrolytic solution, such as salt water or car shampoo, into the pin hole causes insulation failure which is worse than in the conventional voltage environment.